Optical scanning apparatuses are currently in great demand as a result of the rapid development of high speed, low cost imaging devices and the ever decreasing cost of personal computers. Optical scanners, in general, are capable of reflective and/or transmissive scanning of documents, photographs and the like, whereby reflective scanning is used with opaque objects such as photographs and transmissive scanning is used with transparent objects, such as slides and the like. Reflective scanners project light onto the side of the object to be scanned. The reflected light is viewed by an optical sensor which converts the reflected light into digital code. The code is then transmitted to the operator's computer for further processing. Transmissive scanners operate on the same principle except that light is projected through the transparent object being scanned. A great demand exists for high quality, low cost, low maintenance, easy to calibrate, versatile optical scanners capable of both high and low resolution reflective and transmissive scanning.
A conventional scanner typically has scanning platform(s) and a chassis including an optical sensor, a lens assembly, a plurality of mirrors and light source(s), mounted on rails inside the scanner housing and driven by a motor within the housing to scan along the length of the document being placed on the scanning platform. During reflective operation the light source projects light upon the portion of an opaque object within the field of view of the optical sensor, with the scanner chassis moving incrementally lengthwise under a reflective scanning platform and scanning an image line by line. During transmissive operation, another light source, preferably housed under a transmissive scanning platform, within the scanner housing, provides light through a portion of a transparent object within the field of view of the optical sensor. Switching between transmissive and reflective modes may be accomplished by transverse movement of the lens assembly and/or corresponding movement of mirrors within the scanner housing, depending on the nature of object being scanned. Actual switching is controlled by the operator, who programs the scanner for the desired mode of scanning before starting the operation. A scanner which employs movable mirrors only to switch between scanning modes, is described, for example, in U.S. patent application Ser. No. 08/393,112, filed Feb. 21, 1995 (attorney docket: 5592-1016), commonly owned by applicant, and which is incorporated herein by reference. Such transmissive/reflective scanners provide scanning of objects in one resolution only. Thus, the need exists for a versatile optical scanner capable of multi-resolution transmissive and reflective scanning.